Polybenzoxazoles may be formed by the condensation of a carboxylic acid (or its derivatives) with an o-aminophenol. The two sets of functional groups are referred to as A (indicating a carboxyl group affixed to a benzene ring) and B (indicating the amino and hydroxyl groups affixed to the same benzene ring), respectively. An AA monomer is a dicarboxylic acid, and BB and AB designations have corresponding meanings. Two examples of typical AB monomers are 3-amino-4-hydroxybenzoic acid and 4-amino-3-hydroxybenzoic acid.
Polybenzoxazoles are well known rigid polymers, environmentally resistant, and possessing high modulus and high strength properties. They have found utility in the preparation of thermally stable transparent coatings for solar cells that inhibit ionizing radiation, and in molecular composite technology where they are used as reinforcements in blends with other common polymers. However, the lack of an inexpensive process for preparing the AB monomer precursors has been a handicap to their manufacture.
U.S. Pat. No. 4,847,350 describes a rapid process for the formation of heterocyclic aromatic polymers by contacting multifunctional aromatic compounds, such as a diaminobenzenediol, a terephthaloyl halide and/or an aminohydroxybenzoic acid, with a sulfonic acid and a dehydrating agent. These are two-step reactions and result in the 2,5-and 2,6-polybenzoxazoles.
The AB monomer 3-amino-4-hydroxybenzoic acid is prepared from 4-hydroxybenzoic acid by nitration followed by reduction. The hydroxyl groups need not be protected. The AB monomer 3-nitro-4-hydroxybenzoic acid is reduced by aqueous stannous chloride in hydrochloric acid or by catalytic hydrogenation. Of these two possible aminohydroxybenzoic acids, the 3- amino-4-hydroxy isomer is more easily prepared because of ease of nitration.
By contrast, 3-hydroxyanthranilic acid (HAA), otherwise known as 2-amino-3-hydroxybenzoic acid, is a known compound that, so far as is known, has never been used in the preparation of polymers. HAA is a product of the transformation of the biosynthetically prepared DHAA and is the subject of this portion of the invention.
Various chemical syntheses of 2-amino-3-hydroxybenzoic acid have been reported. They are multistep processes and therefore extremely cost intensive.
Biosynthetically prepared monomers offer several advantages to traditional chemical preparations. These advantages include the use of cheap, renewable feedstocks, the capability of producing multifunctional products in a single step with unmatched chemical selectivity, and the effectiveness of synthesis under mild conditions.
The biosynthesis of DHAA was first reported in 1962 by McCormick et al., U.S. Pat. No. 3,038,005. It used a mutant strain of Streptomyces aureofaciens (Sa-652). The conversion of isolated and purified DHAA to anthranilic acid, m-hydroxybenzoic acid, HAA and hexahydro-3-hydroxyanthranilic acid were also outlined in the same report. However, the yield of DHAA by McCormick procedure has not been found reproducible.
In separate studies, DHAA was isolated from fermentation broth of Streptomyces zaomuceticus SF-1836 and was tested for antibacterial and antifungal activities; Betenandt, Liebigs Annalen der Chemi., 602, 61 (1957)]. The precise assignment of the structure of DHAA was later accomplished by Ganem et al., J. Am. Chem. Soc., 106, 2463 (1984), and independently by Walsh et al., J. Am. Chem. Soc., 106, 2443 (1984).
The present invention provides an improved process for producing relatively inexpensive DHAA. It further provides 2,4-polybenzoxazole homopolymers and copolymers made from AB monomer precursors including HAA.